The present invention relates generally to the field of digital data and more particularly, but without limitation, to the encoding of hidden data onto a digital audio or video carrier signal, written to a recording medium, such as an optical disc.
Optical discs have become increasingly popular as an efficient and cost-effective storage medium for digitally stored data. A typical optical disc comprises a circular disc having a recording layer of light reflective material embedded in a refractive substrate. The recording layer is disposed along a plane substantially normal to an axis about which the disc is rotated and stores data in the form of localized pits and lands (also sometimes referred to as xe2x80x9cmarksxe2x80x9d and xe2x80x9cspacesxe2x80x9d) along a continuously extending spiral track. The length of each pit and land corresponds to one of a selected number of data symbols (for example 3T to 11T, with T of a determined length.)
The data symbols are recovered from the disc through the use of a light source (such as a laser) which applies light of a selected wavelength to the rotating disc and a transducer which generates a readback signal indicative of the data in relation to the relative differences in reflectivity of the pits and lands. It is common to separate the relative elevations of the pits and the lands by a distance equal to a quarter wavelength of the applied light so as to facilitate a substantial change in the amount of light reflected by the pits as compared to the amount of light reflected by the lands.
One popular optical disc format is commonly referred to as a compact disc, or CD, which has found widespread use in recent years in computer applications (such as CD-ROM) and in the music recording industry (audio CDs). A CD has an outer diameter of 120 millimeters (4.724 inches) and a data storage capacity of about 650 megabytes (MB).
Another popular optical disc format is commonly referred to as digital versatile disc, or DVD. A DVD can be considered a xe2x80x9chigh-densityxe2x80x9d CD, in that a typical DVD has generally the same dimensions as a CD, but can store about 4.7 gigabytes (GB) of data per recording layer, due to increased data storage densities through reductions in pit/land geometry and improvement in data encoding and recovery techniques. Other optical discs configurations with respective form factors and data storage capacities have been proposed and commercialized.
Due to worldwide consumer demand for the types of information available on optical discs (e.g. software, video, music, etc.), combined with the relative ease with which unauthorized copies of optical discs can be generated, suppliers of optical discs have attempted to implement various copy protection schemes to restrict unauthorized replication of the discs. One type of copy protection involves configuring an optical disc in such a manner that an authorized copy functions properly in a readback system, but an unauthorized copy is prevented from doing so. Another type of copy protection provides for the placement of forensic tracking information on the disc, with this information relating to the source of the disc, the mastering data and so on. Forensic tracking generally does not prevent an unauthorized copy from functioning in a readback system, but based on the presence or absence of the information, a determination can be made whether a particular optical disc is in fact an authorized copy.
Since optical discs are portable record carriers designed to be played in any number of commercially available playback systems, the data on an optical disc are organized in accordance with predefined industry standards to allow the playback systems to function as intended to retrieve the data stored on the disc. Such standards typically organize the data into a number of successive frames with control fields (subcode) for use by the readback system and user data fields for storing the actual user data (such as audio, video, software, etc.). Error correction codes (parity and/or Reed Solomon codes) are also used to provide on-the-fly error correction during readback.
A typical disc will further include so-called xe2x80x9cnullxe2x80x9d or xe2x80x9csilencexe2x80x9d zones to separate other regions of the disc. Such silence zones are of relatively short duration and can be inserted between songs (tracks) on an audio disc and chapters on a video disc, as well as used as lead-in and lead-out zones to identify beginning and ending portions of the disc. Such silence zones may include user data fields, but do not include human audible information (in the case of an audio CD) or human visual visual (in the case of a video disc).
There is a need for improvements in the industry to place hidden data on a carrier signal without compromising the integrity of the signal itself, and it is to such improvements that the present invention is directed.
The present invention is directed the placement of a hidden message, comprising encoded hidden data, in a silence (null) zone of a digital audio or video carrier signal. The hidden data are encoded and substituted for a portion of the existing silence zone signal while maintaining the silence of the zone. Examples of such silence zones include lead-in zones (at the beginning of an optical disc), lead-out zones (at the end of an optical disc), at pause areas (typically 2-3 seconds in length) between adjacent audio selections on an audio disc, and at xe2x80x9cblack screenxe2x80x9d areas between adjacent video selections on a video disc.
By encoding a hidden message in a location where the audio or video content is held below a specific level, the hidden data does not interfere with the normal transmission of the carrier signal and will have a sufficiently low signal to noise ratio to not be visually or audibly detected by an end user. In the preferred embodiment, the hidden data are represented as a binary sequence of a specified bit length, such as 16 bits. An encoding value of xe2x80x9c1xe2x80x9d is assigned a sample value such as xe2x80x9c+1xe2x80x9d and encoding value of xe2x80x9c0xe2x80x9d is assigned a sample value such as xe2x80x9cxe2x88x921xe2x80x9d. The hidden data are encoded by sequencing these two different symbols xe2x80x9cxe2x88x921,+1xe2x80x9d to represent the encoded binary string.
Another aspect of the present invention is the transfer of the hidden data using a recording medium, such as an optical disc. The present invention provides for the mastering of an optical disc with the insertion of this hidden data. During the mastering process in which a glass master is produced, a portion of the silence zone data is extracted and the hidden message is written therein, prior to a signal processing unit receiving the input data. Furthermore, the present invention also provides for the extraction of the hidden data from a normal optical reading device. When the hidden data are extracted, a second output device processes the same output signal from a digital to analog converter which is provided to an output device, such as an audio speaker. Since the hidden message is silent, the hidden data may be detected, extracted and decoded without interfering with normal optical disc reading operations.
The placement of hidden data on an optical disc may serve many different purposes, such as the placement of forensic tracking information on the disc, encoding and writing ownership information into a silence zone. Another use of the hidden data may be for copy protection purposes in which authentication and verification of the disc is performed by a reading device with an internal microprocessor.
Other features of the present invention provide for a higher degree of accuracy in the detection and decoding of the hidden message. The hidden data may be written to a silence zone with a begin synchronization pattern and an end synchronization pattern. Therefore, a reading device monitoring the optical disc output for the hidden message, may only seek to detect the begin synchronization pattern and begin reading thereupon. Another feature of the present invention is encoding the hidden data at a variant frequency, such that the signal may be above or below the range of human detection.